The present invention relates to a method and apparatus for reducing transmission errors in digital subscriber line (DSL) communication systems and, more particularly, to a method and apparatus for reducing or eliminating transmission errors in DSL communication systems caused by baseband ringing by detecting baseband ringing and by reducing the data rate of the DSL communication for the duration of the baseband ringing.
In networks that utilize copper pairs, also referred to as metallic loops or twisted wire pairs, baseband telephony services and DSL services are often provided over the same copper pair. In these types of networks, baseband ringing used to signal incoming call requests can result in the occurrence of bit errors in data being transmitted at the higher data rates associated with xDSL technology. POTS (plain old telephone service) filters, or splitters, are widely used to separate the baseband telephony services, which occupy lower frequencies (i.e., typically below 4 kilohertz (KHz)), from the higher data frequencies associated with xDSL services. POTS filters normally filter out signals above 20 KHz so that noise in the baseband does not adversely affect the xDSL transmissions.
However, even where POTS filters are utilized, it has been determined that baseband ringing can produce impulse noise that causes bit errors to occur in the data being transmitted at the higher, DSL data rates. Accordingly, a need exists for a method and apparatus for reducing the number of bit errors that occur in xDSL data transmissions as a result of baseband ringing.
The present invention provides a method and apparatus for reducing the number of bit errors that occur in xDSL data transmissions as a result of baseband ringing. In accordance with the present invention, it has been determined that the number of bit errors that occur in xDSL data transmissions as a result of baseband ringing can be reduced by lowering the data transmission rate to a predetermined rate while baseband ringing is occurring. This temporary rate reduction results in an increased operating margin during, and slightly beyond, the time interval in which baseband ringing is occurring.
The apparatus of the present invention preferably comprises a filter configured to pass a waveform associated with baseband ringing, an analog-to-digital converter that converts the filtered baseband ringing waveform into a digital signal, and a signal processor that analyzes the waveform to detect baseband ringing. If the signal processor detects the baseband ringing waveform, the signal processor reduces the rate at which DSL data is transmitted over the network. The signal processor also determines when the baseband ringing waveform is no longer present and restores the data transmission rate to the original data transmission rate upon making that determination.
In accordance with the method of the present invention, when a baseband ringing waveform has been detected, the rate of data transmission is reduced to a predetermined rate and a ring timer is set to a preselected time period. When the preselected time period expires, the rate of data transmission is set to the original rate. Preferably, the method of the present invention is performed in software, which is executed by the signal processor. The software preferably comprises two separate routines that are periodically called by the signal processor. One of the routines detects the baseband ringing waveform, sets the ring timer, reduces the data transmission rate to a predetermined rate and returns to the point of execution from which the call to the routine was made. The other routine decrements the ring timer, determines whether the ring timer has been decremented to 0, sets the data transmission rate to the original rate when the ring timer has been decremented to 0, and returns to the point of execution from which the call to the routine was made.
In accordance with the preferred embodiment of the present invention, the apparatus of the present invention is comprised in a data communication equipment (DCE) device (e.g., a DSL modem) located at the central office (CO). The signal processor that performs the method of the present invention preferably is the central processing unit (CPU) of the DCE located at the CO. In addition to the normal tasks performed by the CPU of the DCE, which are well known in the art, the CPU of the DCE executes the two aforementioned routines. In accordance with this embodiment, the DCE located at the CO sends a command to a DCE located at the subscriber premises that informs the subscriber premises DCE of the rate at which data is to be transmitted between the DCE located at the central office and the DCE located at the customer premises. The DCE located at the CO and the DCE located at the customer premises then reduce their respective data rates to the predetermined data rate. When the data transmission rate is to be returned to the original data transmission rate, the DCE of the CO commands the DCE located at the customer premises to transmit data at the original rate, which causes the DCE located at the customer premises to transmit data at the original data transmission rate. The DCE at the CO returns its rate to the original rate and the DCEs at the customer premises and at the CO begin communicating at the original rate.
These and other features and advantages of the present invention will become apparent from the following description, drawings and claims.